The invention relates to a Rake receiver of a radio system using a Code Division Multiple Access (CDMA) method.
In radio systems, diversity methods of different kinds are used for increasing the coverage area and/or capacity of the system. As to this publication, space diversity, i.e. antenna diversity, polarization diversity and multipath diversity are of interest. Space diversity indicates that antennas are positioned sufficiently far from each other to achieve a sufficient decorrelation between signals received via the separate antennas. An interesting kind of polarization diversity is implicit polarization, i.e. a signal is sent on one polarization level, but received by cross-polarized antennas. Multipath diversity refers to diversity created by multipath propagated signal components, this diversity being usable in a system, such as a CDMA system, in which the bandwidth of a signal is much wider than the coherent bandwidth of a channel.
In a CDMA system, a Rake receiver is used for separating multipath propagated signal components at reception. In general, the signal components must then be separated from each other by at least one chip of a spreading code used. The Rake receiver comprises Rake fingers and, in each of these fingers, despreading and diversity combination take place. In addition, the receiver comprises a delay estimator having a matched filter for each antenna branch and an allocation block for the Rake fingers. In the matched filter, a signal, received by a spreading code used for signal spreading, is correlated by different delays, the timing of the spreading code then being changed for instance in steps of one chip. When the correlation is high, a multipath propagated signal component is found and it can then be received at the delay found.
On the radio path, the signal will include not only the desired signal but also noise and interference caused by other users or systems. In systems utilizing diversity, the influence of noise and interference can be decreased for instance by the Maximal Ratio Combining (MRC) method, in which signals received via separate antennas are weighted in proportion to the signal power in the separate antenna branches. However, this method presupposes that the interference of each antenna is independent. This presupposition is not always true in actual cellular radio networks, but it is conceivable that the same interference is present at each antenna.
There is no such restriction on the Interference Rejection Combining (IRC) method. However, the method has been used only in systems utilizing the Time Division Multiple Access (TDMA) method, these systems often being incapable of separating multipath propagated signal components. Herein, an IRC method refers to adaptive beam formation (optimal combination of signals), by which signal power is maximized in proportion to the power of interference and noise, i.e. the Signal-to-Interference-and-Noise Ratio (SINR) is maximized. Now we shall concentrate on the code acquisition block, or delay estimator, of the receiver. It consists of L matched filters and an allocator for Rake finger allocation. The task of the matched filters is to match the spread and scrambled pilot sequence to the complex conjugated antenna signal in order to resolve the delays of the channel impulse response taps. In the Rake finger allocation, the temporal Rake fingers are allocated for the different multipath components of the received signal.
The matched filters can also be implemented as a bank of parallel correlators which carry out the correlation function of the complex conjugated spreading sequence. Each correlator performs despreading procedure which, mathematically, is the calculation of the cross-correlation function between the received signal and the cophasal complex conjugated spreading sequence.
The outputs of the correlators are used for allocating the Rake fingers to demodulate the strongest muitipath components of the received signal. The current method of Rake finger allocation is based on the energy of despread pilot symbols from L antennas. The outputs of despreaders are summed up at each code phase and N temporal Rake fingers are allocated according to the strongest energy of the sum signal. In the WCDMA (Wideband CDMA) concept the delay is estimated from the dedicated physical control channel (DPCCH). The result is averaged over several time slots to get improved estimates for the Rake finger allocation process.
The current Rake finger allocation is optimal in a spatially white interference scenario, in which the interference sources are evenly distributed in the angular domain. In a spatially coloured interference field, a high-powered interference source can decrease the performance of the receiver because the Rake fingers are allocated to the wrong chip delays.
The present invention seeks to provide an improved Rake receiver. According to an aspect of the present invention, there is provided a Rake receiver as specified in claim 1. The preferred embodiments of the invention are claimed in the dependent claims.
The presented optimum combining scheme is capable of placing nulls towards interfering signals. Owing to this, the interference can be suppressed in Rake finger allocation. The number of the wrong Rake finger allocations can therefore be decreased, which improves the performance of the receiver. The receiver can also better track the changing interference field, and the spatial properties of interference field are taken into account in delay estimation.